Limestone rock is composed of medium particle sizes and is derived from calcite. Limestone can be clastic with particles of any size and it can also be chemical.  It is a sedimentary rock composed principally of calcium carbonate (calcite) or the double carbonate of calcium and magnesium (dolomite).  It is commonly composed of tiny fossils, shell fragments and other fossilized debris.  These fossils are frequently visible to the unaided eye on close examination of the stone surface.  Limestone coloration is generally a consistent pure white to off-white.  Often grey, it may also be yellow or brown.

Limestone is very soft density and can range in hardness depending on type.  It can be a soft rock and is easily scratched.  It is porous and will effervesce readily in any common acid.  Limestone does not take polish well and typically has a non-glossy surface with a matte finish.  Limestone’s can be smooth to rough and may vary greatly in texture and porosity from coquina to oolitic.  Microcrystalline limestones have an extremely fine grain.

There are dozens of forms that limestone can take.  They include Fossilferous, Lithographic, Coral rag, Chalk, and Tufa.

The most common forms of limestone are:

Calcarenite composed of sand-sized carbonate grains of calcite.  The Pietra di Bismantova in central Italy is an example of calcarenite formation.
Oolitic Limestone is calcarenite that contains a quantity of oolites.  Oolites are spherical grains shaped like an egg.  The islands of the Lower Keys in Florida are mainly oolitic limestone.
Coquina is a matrix of whole or pieces of seashells loosely cemented by calcite.  Coquina comes from Spanish for cockleshells or shellfish.  Coquina forms the walls of the Castillo de San Marcos in St. Augustine, FL.
Dolomite rock or dolostone is composed primarily of the mineral dolomite, CaMg(CO2)3.¬† Dolomite is essentially a limestone that contains from 5% to 40% magnesium carbonate.¬† ‚ÄúThe Dolomites‚ÄĚ mountain range, part of the Italian Alps, has the largest exposures of dolomite rock on earth.
Microcrystalline limestone contains calcium carbonate (calcite) structures that are so fine that they can be seen only under magnification.  Microcrystalline calcite (CaCO3) has a crystalline texture.
Travertine is limestone deposited by surface waters such as mineral springs, especially hot springs.  It is noted for its holes and is extremely porous or cellular.  The Colonnade of St. Peter’s Square in Rome is built with Italian Travertine

Limestone is widely used in architectural applications for paving, walls, decorative trim and veneer applications. ¬†¬†Many landmarks across the world, including the Great Pyramid and its associated complex in Giza, Egypt, are made of limestone. Kingston, Ontario, Canada has so many buildings constructed from limestone it that it is nicknamed the ‘Limestone City’.¬† Limestone was a very popular building material for train stations, banks and other public structures during the late 19th and early 20th centuries.¬† Many famous buildings in London are built from Portland limestone.¬† In the USA the Bloomington area has been a source of high quality quarried limestone called Indiana limestone.

Marble.  Limestone deposits can undergo metamorphism during major geological events resulting in a recrystallizing as marble.  In the Building Trade, the same stone can be marketed one time as a limestone and, at another time and place, sold as marble.

Bluestone.¬† The common commercial name of ‚Äúbluestone‚ÄĚ is applied to a variety of building stones. ¬†The bluestone that comes from the Shenandoah Valley in the USA, and from the Hainaut quarries in Soignies, Belgium and from quarries in County Carlow, County Galway and County Kilkenny in Ireland is actually limestone.